A well-known and widely used process for producing formaldehyde involves the passing of a mixture of methanol and air over or across a silver catalyst. Such process combines dehydrogenation and oxidation to obtain the formaldehyde, and such is well described in the literature.
In conducting the reaction the methanol-air mixture is contacted with a silver catalyst at elevated temperatures. In most processes, the silver is present as silver crystals in a bed which is on the order of 1 to 4 centimeters thick and through which the air-methanol mixture is passed. The silver crystals generally form a sintered cake after a period of time. It is known, however, to employ the silver catalyst in other forms, such as in the form of a fluidized bed, or on a catalyst support such as silica gel. Use of a bed of silver crystals is preferred. Contact times for the air-methanol mixture with the silver catalyst may vary widely, but will generally be on the order of 0.005 to 0.05 seconds, preferably from 0.007 to 0.03 seconds.
The molar ratio of air to methanol in the mixture contacted with the catalyst should be at least 0.8 moles of air per mole of methanol and is more usually within the range of 0.9 to 1.4 moles of air per mole of methanol. The mixture which is passed over the catalyst is not limited to methanol and air since various diluents have been disclosed in the literature as being suitably present. Also, recycled formaldehyde may be present. Included among diluents which may be present are steam, carbon dioxide, nitrogen and the like. Where very high air to methanol ratios are utilized, the use of a diluent may especially be desired in order to avoid flammable mixtures.
The pressure utilized in conducting the reaction is usually within the range of one to three atmospheres absolute, although higher or lower pressures may be utilized if desired. Relatively high temperatures are involved since the reaction is exothermic overall even though both an indothermic and exothermic reaction are involved. Since the reaction is exothermic overall no heat addition to the reaction zone is necessary. The methanol does, however, have to be preheated to a temperature at least sufficient to vaporize such, and, preferably both the air and the methanol, and any diluents, are preheated to some degree. Generally the air and any diluent will be preheated separately and apart from the methanol. The methanol-air feed mixture passed to and contacted with the silver catalyst is generally at a temperature within the range of about 70.degree. C. to 125.degree. C., preferably 75.degree. C. to 100.degree. C.
Due to the fact that the reaction is overall exothermic in nature, the temperature of the catalyst bed will generally be within the range of 475.degree. C. to 675.degree. C., and preferably such is within the range of 550.degree. C. to 625.degree. C. The temperature of the catalyst bed can be regulated by controlling preheat temperature of the feed, and by controlling the ratio of air to methanol.
The foregoing background discussion is very general in nature and those skilled in the art are aware of numerous literature references to variations on the basic process. Despite the fact that the silver catalyst method has been thoroughly researched and in commercial use for years, improvements in the process are still being sought. It is thus an object of the present invention to provide an improvement in the process for the production of formaldehyde by the silver catalyzed reaction of methanol and air. It is a particular object of the present invention to provide an improvement in such process whereby a greater selectivity to formaldehyde may be attained. Additional objects will become apparent from the following description of the invention.